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Antibiotics are a group of medicines that are used to treat infections caused by some germs (bacteria and certain parasites). A parasite is a type of germ that needs to live on or in another living being (host). Antibiotics do not work against infections that are caused by viruses - for example, the common cold or flu.

Antibiotics are normally only prescribed for more serious bacterial infections, as many infections get better on their own. Correct use of antibiotics is absolutely essential to help reduce antibiotic resistance. Germs become resistant to antibiotics over time, which then makes them less effective.

The World Health Organization (WHO) says "the world urgently needs to change the way it prescribes and uses antibiotics" as antibiotic resistance is a major global threat.

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What are antibiotics?

Antibiotics are a group of medicines that are used to treat infections. Antibiotics are sometimes called antibacterials or antimicrobials. Antibiotics can be taken by mouth as liquids, tablets, or capsules, or they can be given by injection. Usually, people who need to have an antibiotic by injection are in hospital because they have a severe infection. Antibiotics are also available as creams, ointments, or lotions to apply to the skin to treat certain skin infections.

It is important to remember that antibiotics are only prescribed to treat infections and are not effective against viruses. Antibiotics simply do not work against infections that are caused by viruses (for example, the common cold or flu), or fungi (for example, thrush in the mouth or vagina), or fungal infections of the skin.

Microbes, Germs and Antibiotics discusses the different types of germs which can cause infections.

Occasionally, a viral infection or minor bacterial infection develops into a more serious secondary bacterial infection. In this case, antibiotics would be needed.

Types of antibiotics

There are various antibiotics available and they come in various different brand names. Antibiotics are usually grouped together based on how they work. Each type of antibiotic only works against certain types of bacteria or parasites. This is why different antibiotics are used to treat different types of infection. The main types of antibiotics include:

As well as the above main types of antibiotics, there are a number of other antibiotics that specialist doctors or hospital doctors may prescribe for more uncommon infections such as tuberculosis (TB).

The rest of this leaflet only discusses antibiotics that your GP may prescribe.

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How do antibiotics work?

Some antibiotics work by killing germs (bacteria or the parasite). This is often done by interfering with the structure of the cell wall of the bacterium or parasite. Some work by stopping bacteria or the parasite from multiplying.

When are antibiotics usually prescribed?

When do I need antibiotics for a sore throat?

Antibiotics are normally only prescribed for more serious infections with germs (bacterial and some parasitic infections).

Most common infections are caused by viruses, when an antibiotic will not be of use. Even if you have a mild bacterial infection, your immune system will usually be able to clear it without the need for antibiotics. For example, antibiotics usually do little to speed up recovery from most ear, nose, throat and sinus infections that are caused by bacteria.

So, do not be surprised if a doctor does not recommend an antibiotic for conditions caused by viruses or non-bacterial infections, or even for a mild bacterial infection.

However, you do need antibiotics if you have certain serious infections caused by bacteria, such as meningitis or pneumonia. In these situations, antibiotics are often life-saving. When you are ill, doctors are skilled at checking you over to rule out serious illness and to advise if an antibiotic is needed. Urine infections also commonly need antibiotics to prevent spread to the kidneys.

Antibiotics can also be prescribed to treat acne - a less serious condition. For acne, antibiotics can be taken by mouth or applied directly to the skin.

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Which antibiotic is usually prescribed?

The choice of antibiotic mainly depends on which infection you have and the germ (bacterium or parasite) your doctor thinks is causing your infection. This is because each antibiotic is effective only against certain bacteria and parasites.

For example, if you have pneumonia, the doctor knows what kinds of bacteria typically cause most cases of pneumonia. He or she will choose the antibiotic that best combats those kinds of bacteria.

There are other factors that influence the choice of an antibiotic. These include:

  • How severe the infection is.

  • How well your kidneys and liver are working.

  • Dosing schedule.

  • Other medications you may be taking.

  • Common side-effects.

  • A history of having an allergy to a certain type of antibiotic.

  • If you are pregnant or breastfeeding.

  • Pattern of infection in your community.

  • Pattern of resistance to antibiotics by germs in your area.

Even if you are pregnant or breastfeeding there are a number of antibiotics that are safe to take.

How to take antibiotics

It is important to take antibiotics in the correct way. If you do not, this may reduce how well they work. For example, some antibiotics need to be taken with food and others should be taken on an empty stomach.

If you do not take your antibiotics in the right way it will affect how much of them get into your body (their absorption) and therefore they may not work as well. So, follow the instructions given by your doctor and on the leaflet that comes with the antibiotic you are prescribed.

Always take the entire course of antibiotics as directed by your doctor. Even though you may feel better before your medicine is entirely gone, follow through and take the entire course. This is important for your healing. If an antibiotic is stopped in mid-course, germs (bacteria) may be partially treated and not completely killed. Bacteria may then become resistant to that antibiotic.

Overuse of antibiotics has led to some bacteria changing their form or structure (mutating) and becoming resistant to some antibiotics, which may then not work when really needed. For example, meticillin-resistant Staphylococcus aureus (MRSA) is a bacterium that has become resistant to many different antibiotics and is difficult to treat. Other bacteria produce chemicals called enzymes such as extended-spectrum beta-lactamases (ESBLs) which allow them to be resistant to certain antibiotics.

Resistance to antibiotics has become a major health threat globally, as antibiotics are life-saving for serious illnesses. So if your health professional says antibiotics are not needed for your condition, do not ask for them. Your health professional will explain in what circumstances you might need antibiotics and what symptoms should prompt you to return for another check.

If your illness has changed, or you have developed a secondary infection, antibiotics may be needed after all. Health professionals follow guidelines to help reduce unnecessary use of antibiotics.

Side-effects of antibiotics

It is not possible in this leaflet to list all the possible side-effects of each antibiotic. However, as with all medicines, there are a number of side-effects that have been reported with each of the different antibiotics. If you want more information specific to your antibiotic then you should read the information leaflet that comes with the medicine.

Most side-effects of antibiotics are not serious. Common side-effects include:

Less commonly, some people have an allergic reaction to an antibiotic, for example an itchy rash. Severe allergic reactions can be fatal, but this is extremely rare.

Antibiotics can kill off normal defence bacteria which live in the bowel and vagina. This may then allow thrush or other bad bacteria to grow.

You should tell your doctor if you have any of the following side-effects:

Speak to your pharmacist if you have any vaginal itching or discharge when taking an antibiotic (signs of thrush)



Some antibiotics may interact with other medicines that you might take. This may cause reactions, or reduce the effectiveness of one or other of the treatments. So, when you are prescribed an antibiotic you should tell a doctor if you take other medicines.

Oral contraceptive pill

In the past it was recommended that, if you were taking antibiotics and were also taking the pill, you should use additional contraception. This is no longer the current recommendation after more recent evidence has been reviewed.

Antibiotics (other than one called rifampicin) do not interfere with the effectiveness of the pill. You should continue taking your pill as normal if you also need to take any antibiotics.

Can you buy antibiotics?

In the UK they are generally only available from your pharmacist, with a doctor's prescription. In some parts of the UK it is possible to get a very limited selection of antibiotics directly from your pharmacist, for example to treat a urinary tract infection or skin infection. In some other parts of the world they are freely available over the counter. However, to reduce the problem of resistance due to inappropriate use of antibiotics, it is best to always obtain medical advice before buying antibiotics.

How long do you take antibiotics for?

The length of treatment varies a lot. It depends on what kind of infection you have, how severe it is and how quickly you get better after starting treatment. Treatment can be:

  • For just a few days ('water' infection - urinary tract infection (UTI)).

  • For one or two weeks (pneumonia).

  • For a few months (bone infections).

  • For many months (acne).

Who cannot take antibiotics?

It is very rare for anyone not to be able to take some type of antibiotic. The main reason why you may not be able to take an antibiotic is if you have had an allergic reaction to an antibiotic in the past. Even if you have had an allergic reaction to one antibiotic, your doctor or health professional will be able to choose a different type of antibiotic, which you will be able to take.

If you are pregnant, there are certain antibiotics you should not take, but your health professional will be able to advise on which one is suitable if an antibiotic is needed. If you are on some medication, certain antibiotics may need to be avoided, or your regular medication stopped whilst you take the antibiotic. As above, when prescribed an antibiotic, make sure the prescriber knows about any other medication you take.

Further reading and references

Article history

The information on this page is written and peer reviewed by qualified clinicians.

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